Techniques... For riding up hills

I'm definitely a spinner with the exception of little ramps where I'll pop ups for a few pedal strokes to get the extra oomph and then sit down again.

There are pitfalls with spinning too much though - you start spending energy on moving your legs around rather than actually moving the pedals, if that makes sense, and if you never really put any pressure through the pedals you end up with great lungs and great muscular reaction, but no strength.  As with all things - need a bit of both!

As I understand it, the most efficient way to climb is to let your gears do all the work. Ride just as you would on the flat - i.e. seated, and use your gears to stick to your normal cadence. This might include shifting up/down as the gradient fluctuates.

You can get a bit more raw power by standing up and using a slightly harder gear, but it won't be as efficient so it's hard to keep up for long and you'll burn yourself out if you try to do it too much.

I think it depends on the length of the climb.  If it's just a ninety second to two minute rise, standing and using momentum to stay on top of a big gear gets good results for me.  It allows me to stay at a far higher power than would otherwise be sustainable.  With four to ten minute climbs, I tend to use a mix of both seated and standing but stand more where the gradient inreases.  Anything longer is predominatly seated for me with a bit of standing for stretching or to quicken the pace when it's less steep.  

What really helps is getting to know a climb and what gears work best where and when best to change them or where best to just use bigger gears and muscle your way through a section.

I've read recently that "spinning is more efficient" is more for the pros, more for people doing long hard rides, day after day, more about recovery and being able to go out and do it again the next day.

My last bike had a 53-39 with a 23-12 cassette, and I noticed all my friends on long hills who could spin would ride away from me whilst I was standing and hauling on the bars.  When the cassette wore out, I replaced it with a 25-12, and noticed they weren't getting away as quickly, and then when I got a 27-12 I was almost able to hang on.

I think you should make use of all the gears you have, and use them efficiently.  It's a balance between hanging on to momentum, and not bogging down.  If you shift under load, you're more at risk of something going wrong mechanically, which can also cause you to crash.  

Sit and spin on the roads, tend to stand and climb offroad

If it's a very short hard effort I'll stand when road riding but that's more to do with not shifting than anything else.

I know you shouldn't shift under power but I do it and I'm a big heavy rider. It's all in the timing, just very briefly soft pedalling whilst it shifts, and only dumping one gear at a time.

I'm more of a stander, but I'm trying to train myself to remain seated going up hills as it should hopefully be more efficient. For short hills it's more fun to just stand and stomp your way up them but with longer hills I end up slowly stomping which ends up being disappointingly slow.

One advantage to switching between spinning and stomping is that you're putting strain on different parts of your muscles so it's almost like having a rest.

I generally don't stand up for long periods when i'm climbing outdoors, but i find it much easier on the turbo to stand, not sure why. it's generally accepted that seated and spinning is the more efficient way to climb overall, but there's a lot of personal preference in the mix too, which is why you'll see pros that do both. 

Sit and spin for me, with properly low gears.

weird thing is Ive never got the hang of pedalling whilst standing up, it just feels wrong like Im not keeping balance on the bike properly which inevitably leads to a foot down stop moment. so I always spin though fortunately there arent too many hills around where I live as really dont like hills much

Sometimes, being in the correct mindset can help you up the hills.
If you think you're going stop part way up the hill and walk the rest. You're more likely to fail, than if you're absolutely determined to reach the top.

Think of your legs like some massive steam engine powering the wheels, and not an AAA battery powering a weedy electric motor 😜

Pace yourself, first half easier second half harder

I vaguely remember reading about a study into this. The conclusion was that standing produces more power (so quicker) but sitting is more efficient.

I always sit and spin, worried about wrecking my knees.